Dinosaurs Come Through In The Clutch

In the last few months, a lot has been going around about a pretty interesting topic—dino sex. Besides the mechanics of dinosaur sex and reproduction (some people love talking about that)- there are some more interesting questions to consider. For example, how many eggs did most dinosaurs lay? Some have suggested that enormous dinosaurs, such as sauropods, had many offspring per year, and this contributed to their gigantism. Previously, the only things we were able to discern about dinosaur reproduction were what was uncommonly preserved in the fossil record—such as complete fossil egg clutches, very rarely with a brooding parent on top. These finds are amazing and interesting, but because they are so rare, it is difficult to use these fossils to make broad generalizations about reproduction in different groups of dinosaurs.

In a paper out yesterday in PLOS ONE, authors Jan Werner and Eva Maria Griebeler looked at extant birds and reptiles in order to better understand how many eggs some dinosaurs may have laid during the year. They constructed a really cool allometric regression model based on modern species and found a close correlation between body mass, egg mass, clutch mass, and annual clutch mass.

When there is a clear idea of how extant animal size relationships function, the same equation can be applied to dinosaurs using extant phylogenetic bracketing. In other words, non-avian dinosaurs will be plugged into the same allometric equation as their closest ancestor. Theropods were plugged in to the “bird model” because they are most closely related to modern birds. Hadrosaurs and sauropodomorphs were plugged into the “reptile model” because the authors decided these dinosaurs were less related to birds than to reptiles since they were outside Therapoda. The results indicate that theropods probably only had one clutch per year, but hadrosaurs and sauropodomorphs may have had several. Surprisingly, using these calculations, most of the dinosaurs studied had less than 200 eggs per year. Enormous 75,000kg sauropods had around 400 eggs per year, which is less than extant sea turtles that can lay over 500 eggs per year!

But why did these different types of dinosaurs have different egg-laying strategies? Well, that is fairly open to interpretation. It is possible that the environments these animals lived in were so different that theropods were able to get by on one clutch per year, while other dinosaurs may have lived in environments where their clutches did not experience good survival rates. It is definitely clear though that there was a shift at some point during the evolution of non-avian dinosaurs to birds relating to egg and clutch size. Theropods had larger eggs than sauropodomorphs relative to body size, but had fewer eggs in their clutch. Modern birds have the largest eggs compared to body size of all, but by far the smallest clutch size, only laying 2.2-4.5 eggs per clutch. It will be interesting to explore and hypothesize about the drivers of this trend.

I always enjoy elegant studies like this one that utilize mostly published data to make new inferences about extinct animals. There are already so many data out there that paleontologists can utilize without relying on new fossil finds, so get (data)mining!